My son wanted a Lego party for his 5th birthday. Since there is not a basic lego party theme we did it ourselves.  For invitations I bought 4x6 red note cards and rounded the corners with a corner cropper (not necessary). I strategically printed LEGO in a darker red on matching red card stock.  I then used a 1 circle cutter to cut circle out of red card stock so that each circle would have LEGO in the center. I used foam squares to attach 6 circles to the front of the notecard (so that the circles would be elevated) and voila a Lego was created. On the inside (in classic LEGO font)it read... Luke's turning 5 so you gotta help him build a LEGO birthday bash that is mega fun filled! We hand delivered the enveloped so on the outside we used large inkjet mailing labels with each guest name (insted of the word LEGO) in the classic yellow & red with black border "lego logo".  For the cake... I used a large Wilton sheet cake pan and cut it into 3 equal parts. I staggered 2 of the pieces for the base and placed the last on top as though I were building with Legos. I made mini cup cakes and placed them on top to transform the cakes into LEGO bricks.  I iced one brick blue one green and one yellow. On the cake platform I built the word LEGO (from legos) and placed it next to the cake.  Table ware... I used primary yellow blue green and red plates napkins forks and table cloths.  Food... Basic snacks in primary color bowls.  The kids favorite... ICE CREAM SUNDAES!  I use plastic cups and dip one large scoop into each a couple of days prior.  Cover with Press and Seal store in the freezer and when it's time for ice cream there's no messy scooping.  While I put the cake on plates the kids get to add topping to their ice cream. We had mini m&m's marshmallows sprinkles cherries chocolate sauce and whipped cream. I have an adult squirt the whipped cream and "supervise" the sundae table.  Treat bags... I used primary color (yellow blue green and red) "lunch" type sacks and added the guest names to the outside with primary color foam letters. I closed the sacks with cute primary colored clothes pins that I found in the stationary department.  Entertainment... I got a clear PLASTIC jar and filled it with LEGOs and the kids had to guess between 1-300 (because of the ages) and the winner got a small Lego set. For our pinata I bought one of the Large Basic Lego sets and used those legos and candy in the pinata.   Fun extras... I used individual LEGOs for confetti on the tables. Kellogg makes LEGO fruit snacks.  The packages were cute scattered on the tables as well as fun for treat bags.  The basic Lego set that I bought for the pinata came in a LARGE red brick. I tied yellow blue green and red balloons to it for a great center piece.  I used a large blue tarp (from home improvement store) anchored in the ground instead of a table on the lawn for the kids to eat and open presents... when they are done you can just hose it off:)  All of the primary colors made for a really festive look! It was a ton of fun. "